It is well known in the art that there are several technologies for screening bulk material composed of products such as moulds, composts, wood residues, aggregates, etc. These technologies include the use of rotating screeners, star screeners, vibrating screeners, etc. The rotating screeners and the star screeners are much more performant with organic products, such as moulds, composts and wood residues for example, for obtaining end products of small granular sizes (½″ and less, for example) and at high production rates (approximately 150 cubic yards/hour and more, for example). However, these types of screeners are not compatible with inputs of great dimensions such as big rocks, big wooden pieces or big cement blocks, because the latter can easily damage the screens of the rotating screeners or damage the stars and the shafts of the star screeners.
Also known in the art are vibrating screeners which can be used for accepting the above-mentioned larger-sized products. These types of screeners are known to be efficient for screening aggregates. However, their production capacity for obtaining end products of small granular sizes from organic materials is fairly limited. Also, the screens or the perforated plates used with these vibrating screeners often get plugged up when the bulk material to be screened is humid.
In order to avoid the damaging of their rotating screeners or star screeners which are mostly used for screening organic material, several manufacturers will often use a vibrating screen placed over the material reserve used for feeding their rotating or star screeners. This vibrating screen is primarily used for carrying out a primary screening of the inputs of great dimensions in order to then direct the bulk material, free of large debris, into the rotating screener, or star screener. However, in the prior art, the direction of flow of the bulk material onto the vibrating screen is perpendicular to that of the reserve and that of the rotating screener, thus is perpendicular to the longitudinal axis of the screening unit machine. This limits drastically the length allowable for the vibrating screen due to the maximal dimensions allowed for the screening unit for travelling on the road. In fact, the length of the screener is then limited to the width of the screening unit machine which is itself limited to 8′6″ or 8″3″ in most countries in order to be legally allowed to travel on the roads.
Since the length of the vibrating screener or of the vibrating screen is limited, the amount of material that can be unloaded therein with a loader or an excavator is also therefore limited. In fact, if the amount of material unloaded is too great, it causes an overflow of bulk material outside the vibrating screener at the bottom thereof because the retention time of the material is not sufficient enough for it to flow completely through the screen. An important amount of small-sized and medium-sized product finds itself thus with the large-sized products of the bulk material. One must thus decrease the amount of bulk material unloaded onto the vibrating screener or the vibrating screen in order to obtain a suitable classification. Furthermore, the debris of great dimensions slide to the bottom of the vibrating screen, close to the location where the loader must position itself for unloading the bulk material on the same. The operator of the loader must thus clean this area at frequent intervals in order to be able to feed the machine in a suitable and safe manner. All of the above factors lead to a decrease in screening productivity.
Furthermore, there exist vibrating screeners provided with two stages of screening which enable to accept inputs of great dimensions at the upper stage and carry out a selection of precise granular size at the lower stage. However, the capacity of production of products in bulk and the capacity of screening of humid products is not as important with this type of screener as with star screeners or rotating screeners.
Known to the applicant are the following U.S. and foreign patents which describe different screening processes and apparatuses: U.S. Pat. Nos. 517,724; 2,115,110; 2,366,222; 2,703,649; 2,864,561; 3,322,354; 4,256,572; 4,363,725; 4,861,461; 4,956,078; 4,983,280; 5,097,610; 5,100,537; 5,106,490; 5,120,433; 5,234,564; 212642 (Australia); 64987 (Ireland); 74896 (Ireland); 285 882 (Germany); 1,553,667 (London); 1488026 (U.S.S.R.).